A typical inkjet printer reproduces an image by ejecting small drops of ink from a printhead containing ink nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. Inkjet printers typically reproduce color images by using a set of color inks, usually cyan, magenta, yellow, and black. It is well known in the field of inkjet printing that if ink drops placed at neighboring locations on the page are printed at the same time, then the ink drops tend to flow together on the surface of the page before they soak into the page. This can give the reproduced image an undesirable grainy or noisy appearance often referred to as “coalescence”. It is known that the amount of coalescence present in the printed image is related to the amount of time that elapses between printing adjacent dots. As the time delay between printing adjacent dots increases, the amount of coalescence decreases, thereby improving the image quality. There are many techniques present in the prior art that describe methods of increasing the time delay between printing adjacent dots using techniques referred to as “interlacing”, “print masking”, or “multi-pass printing”. These methods often involve advancing the paper by an increment less than the printhead width for each printing pass. As a result, successive passes or “swaths” of the printhead overlap, which has the additional advantage that it can help to reduce one-dimensional periodic artifacts referred to as “bands” or “banding” that can result due to clogged or misdirected ink nozzles. See, for example, U.S. Pat. Nos. 4,967,203 and 5,992,962. The term “print masking” generically means printing subsets of the image pixels in multiple partially overlapping passes of the printhead relative to a receiver medium.
Another attribute of modern inkjet printers is that they typically possess the ability to vary (over some range) the amount of each ink that is deposited at a given location on the page. Inkjet printers with this capability are referred to as “multitone” inkjet printers because they can produce multiple density tones at each location on the page. Some multitone inkjet printers achieve this by varying the volume of the ink drop produced by the nozzle by changing the electrical signals sent to the nozzle or by varying the diameter of the nozzle. See for example U.S. Pat. No. 4,746,935. Other multitone inkjet printers produce a variable number of smaller, fixed size droplets that are ejected by the nozzle, all of which are intended to merge together and land at the same location on the page. See for example U.S. Pat. No. 5,416,612. These techniques permits the printer to vary the size or optical density of a given ink dot, which produces a range of density levels at each location, thereby improving the image quality.
Another common way for a multitone inkjet printer to achieve multiple density levels is to print a small amount of ink at a given location on several different passes of the printhead over that location. This results in the ability to produce a greater number of density levels than the nozzle can fundamentally eject, due to the build up of ink at the given location over several passes. See, for example, U.S. Pat. No. 5,923,349.
In U.S. Pat. No. 5,790,150, Lidke et al. disclose a method where multiple passes are made over the page while fractionally advancing the page. In each pass, the pattern of dots in the data swath is constructed with sufficient spacing between the dots such that the printhead can be scanned across the page at a velocity that is higher than the firing frequency limit of the nozzles.
In U.S. Pat. No. 6,310,640, Askeland discloses a print masking method in which nozzles at the ends of the printhead print with lower duty than nozzles near the center of the printhead, thereby reducing the possibility of banding artifacts occurring at the boundaries between successive printed swaths.
In U.S. Pat. No. 6,206,502 Kato et al. also discloses a method for reducing the duty for nozzles at the ends of the printhead. This method involves using a page advance which is smaller than the number of nozzles in the printhead divided by the number of passes, so that there is a region at the ends of the printhead where the passes overlap for an additional pass. The goal of this is to hide artifacts that can result at the boundaries of the printhead due to page advance errors, etc. Vinals et al disclose a similar method in U.S. Pat. No. 6,375,307 for a single-pass printing configuration. These print masking methods are sometimes referred to as “fractional print masking” in the literature.
In U.S. Pat. No. 6,238,037, Overall et al. disclose a print masking method for a multilevel inkjet printer in which the print mask contains a set of threshold values. A dot will print at a given location on a given pass if the multitone code value for that pixel is greater than the threshold for that pass. This method requires that if a dot gets printed at a given pixel on pass N, then it also must receive dots on passes 0 through N−1.
In U.S. Pat. No. 6,454,389, Couwenhoven et al. disclose a print masking method suitable for multilevel inkjet printers that can produce multiple sized ink drops.
In U.S. Patent Application Publication No. 2007/0201054, which is incorporated herein by reference, Billow et al. disclose a print masking that utilizes a print mask having a plurality of mask planes, each mask plane corresponding to a multitone code value. This approach has the advantage that dot patterns printed in response to different multitone levels can be independent from each other.
The method of Billow et al will now be described in more detail to illustrate print masking. Turning to FIG. 1, a typical inkjet printer system is shown in which an image preprocessor 20 receives a digital image from a host computer 10, and performs standard image processing functions such as sharpening, resizing, color conversion, and multitoning to produce a multitoned image signal i. The multitoned image signal i is composed of a set of color data planes hereinafter referred to as color channels. Each color channel corresponds to a particular colorant in the printer, such as the cyan, magenta, yellow, or black inks used in a typical inkjet printer. The data including each color channel is a two dimensional array (width=w, height=h) of individual picture elements, or “pixels”. The pixel's location in the image is specified by its (x,y) coordinates in the array, where 0≦x≦w−1 and 0≦y≦h−1. The x location of the pixel is also referred to as the pixel column number, and the y location of the pixel is referred to as the pixel row number. The term “signal” is used to generically refer to the array of pixels having digital code values that form the image.
A swath data generator 30 then receives the multitoned image signal i and generates a swath data signal s, which controls the volume of ink printed by an inkjet printhead (or printheads) 40. The process of print masking is contained within the swath data generator 30. Prior to multitoning, each pixel contains a numeric code value (typically on the range {0,255}) for each color channel that indicates the amount of the corresponding colorant to be placed at the given pixel's location in the image. After multitoning (at the output of the image preprocessor 20), the image is represented by multitone code values, where the range of pixel code values has been reduced to match the number of density levels that the inkjet printer can produce. For binary inkjet printers, the possible multitone code values will be either 0 or 1, indicating whether to print 0 or 1 drops of ink. Multitone inkjet printers will accept multitone code values on the range {0,N−1}, where N is the number of possible multitone code values, and is normally the number of density levels (or number of drops) that the multitone inkjet printer can produce at a given pixel.
Turning now to FIG. 2, the details of the swath data generator 30 are shown. A “swath” of data is defined as the dot ejection data that is required during one motion of the printhead across the page. In FIG. 2, according to the method of Billow et al., a print mask for a given color contains a set of mask planes 50, 52, 54, 56, each of which has a Mw×Mh array of individual mask elements 60.
Often, the mask height Mh will be equal to the number of nozzles in the printhead, although this is not a fundamental restriction, and a mask height of lesser or greater value can be used. One of the mask planes is selected for a given pixel according to the multitone code value of the multitoned image signal i, as shown in FIG. 2. A pixel column index xm and a pixel row index ym are computed according to the following equations:xm=x % Mw  (1)ym=y % Mh  (2)where x is the pixel column number and y is the pixel row number of the current pixel being processed, Mw is the mask width, Mh is the mask height, and the “%” symbol indicates the mathematical modulo operator. The value of the swath data signal s is then determined by selecting a mask element 62 from the chosen mask plane according to:s=MaskPlane(i,xm,ym)  (3)
Turning now to FIG. 3, an example mask plane 70 is shown. In the mask plane 70, each of the individual mask elements 80 can be one of two values: a first value (0) indicating that no ink drop is to be ejected, and a second value (1) indicating one drop of ink is to be ejected. Thus, if the mask plane 70 corresponds to multitone code value 1, and a uniform 8×8 input image of multitone code value 1 was input to the swath data generator 20, then a dot pattern indicated by the mask elements having value “1” in the mask plane 70 would be printed in one pass of the printhead. For purposes of illustration, the mask plane 70 is shown as having a mask width and mask height of 8, although one skilled in the art will recognize that a mask of any arbitrary size can be used. Generally, mask sizes will be significantly larger than this, with the mask height typically being equal to the height of the printhead.
Turning now to FIG. 4, the dot patterns resulting from three subsequent passes of an inkjet printhead having 8 nozzles in response to a uniform 8×8 input image of multitone code value 1 are shown. In this example, the print mask used has the mask plane 70 of FIG. 3 set to correspond to multitone code value 1, and the receiver media is advanced by four raster lines between each pass of the printhead. Since the input image has a uniform field of multitone code value 1, mask plane 70 will be selected for every pixel in the 8×8 image, and the pattern of dots printed in each of the three successive swaths will correspond to the pattern of 1s in the mask plane 70. The resulting swath patterns 90, 92, 94 are shown offset horizontally from each other, and the resulting pattern of ink dots 96 is shown, produced by overlapping the individual swath patterns. Note that in regions where two successive print passes overlap, every pixel location has received one drop of ink, which corresponds to the desired output for the 8×8 input image of multitone code value 1. Thus, the print mask shown in the example is appropriate for use in a “2-pass” printmode, meaning that two passes of the printhead are required for the desired final dot patterns to be printed. This also means that the mask plane 70 is designed such that the top half and bottom half of the mask, when overprinted on two subsequent print swaths, will produce the desired number of ink drops at each pixel. In this case, this implies that the top half and the bottom half of the mask plane 70 are complementary, such that a single ink drop will be printed at each location.
Often inkjet printers are configured to print in a bi-directional print mode, where ink is applied as the printhead moves in both rightward and leftward directions. A common problem with inkjet printers that utilize bi-directional multi-pass printing is that they can be susceptible to banding artifacts caused by differences in the order of ink laydown and the timing between ink laydown on different passes. These differences can cause systematic variations in the produced color due to interactions between the ink and media. For example, consider FIG. 5, which illustrates a bi-directional 2-pass print mode. A printhead having an associated print mask 100 first moves from left-to-right, printing ink in a first swath 101 (indicated by a pattern of upward sloping diagonal lines). The paper is then advanced by half of the printhead height and the printhead makes a second pass over the paper, this time moving from right-to-left in a second swath 102 (indicated by a pattern of downward sloping diagonal lines). The paper is then advanced again, and the printhead prints a third swath 103 (indicated by a pattern of vertical lines), moving from left-to-right. The print mask 100 is labeled with two sections, 1 and 2. Section 1 corresponds to the part of the printhead that prints on the media the first time it passes over a given region of the page. Likewise, section 2 corresponds to the part of the printhead that prints on the media the second time it passes over a given region of the page.
It can be seen that there are differences in timing between ink laydown on different print passes, both across the page, as well as down the page. For example, consider a first overlap region 104 where ink is first applied during the rightward first swath 101, and then during the leftward second swath 102. In a left portion of the first overlap region 105 there will be a relatively long time delay between the times that ink is applied on the first swath 101 and the second swath 102. This is due to the fact that the printhead must travel all the way across the page, then turn around and come all the way back across the page. However, in a right portion of the first overlap region 106 there will be a relatively short time delay between the times that ink is applied on the first swath 101 and the second swath 102. This is because the printhead needs to travel a shorter distance before it turns around and comes back. The reverse is found to be true in a second overlap region 107 where ink is first applied during the leftward second swath 102, and then during the rightward third swath 103. In a left portion of the second overlap region 108 there will be a relatively short time delay between the swaths, whereas in a right portion of the second overlap region 109 there will be a relatively long time delay between the passes.
The differences in the time delays, both across the page and from swath-to-swath, can result in significant differences in the characteristics of the reproduced image. When there is a longer time delay between passes, the ink applied during the first pass will have a longer time to dry or soak in to the paper. This can result in noticeable differences in the density of the printed region. Additionally, there can also be noticeable differences in the image structure characteristics. For example, coalescence artifacts, as well as surface characteristics such as gloss and haze, are often observed to be a function of the timing between when neighboring ink drops are applied. Typically, the swath-to-swath differences at a given horizontal position in the image are much more objectionable than the variations across the page. This is because the swath-to-swath differences produce a periodic artifact where the image characteristics vary for alternating swaths. These artifacts are sometimes referred to as bi-directional banding artifacts since they are inherently related to bi-directional multi-pass print modes. The magnitude of these artifacts is quite dependent on the characteristics of the particular ink and media used in the inkjet printer, as well as print mode attributes such the amount of ink, the number of passes and the printing speed. The magnitude of the bi-directional banding artifacts can even be affected by the size of the image since this can have an effect on how long it takes the printhead to travel across the page. For many combinations of ink, media and print mode, the bi-directional banding artifacts have been found to be quite objectionable.
Bi-directional banding artifacts can be even more severe for the case of printing color images with multiple color inks. Consider the case of a color inkjet printer using cyan, magenta, yellow and black inks. Typically, each ink will be printed using a different column of nozzles in the printhead. Therefore, as the printhead moves back and forth across the paper, there will also be differences in the order that the different color inks are applied. For example, consider the case where a uniform blue image region is to be printed using equal amounts of cyan and magenta inks. If the cyan nozzles are located to the right of the magenta nozzles in the printhead, they will be applied before the magenta drops on a rightward swath, but after the magenta drops on a leftward pass.
Consider the case where a blue image region is printed using the 2-pass configuration shown in FIG. 5. In the left portion of the first overlap region 105 the ink will be applied as cyan/magenta/long delay/magenta/cyan. However, in the right portion of the first overlap region 106 the ink will be applied as cyan/magenta/short delay/magenta/cyan. Similarly, in the left portion of the second overlap region 108 the ink will be applied as magenta/cyan/short delay/cyan/magenta, and in the right portion of the second overlap region 109 the ink will be applied as magenta/cyan/long delay/cyan/magenta. Therefore, at a horizontal position on the left edge of the page, the image will alternate back and forth between regions that vary in both the order of ink laydown and the timing between ink laydown on different passes. This can cause objectionable periodic bi-directional banding artifacts that vary in both color (lightness, hue and/or chroma) and image structure. Generally the magnitude of the bi-directional banding will decrease towards the center of the page where the contributions to the bi-directional banding that result from differences in the timing between ink laydowns will be negligible, leaving only the contributions that result from differences in the order of ink laydown. The magnitude of the bidirectional banding will then increase again for horizontal positions toward the right side of the page, although the phase of the banding artifacts that result from differences in the timing between ink laydowns will be reversed relative to the left side of the page. The magnitude of the bi-directional banding artifacts is often much more objectionable for image regions printed with two or more colored inks than it is for image regions using only a single ink since the reproduced color can be strongly influenced by which ink drops are printed on top.
A variety of methods have been proposed to alleviate the objectionable bidirectional banding artifacts. One solution is to use only uni-directional print modes where ink is only printed when the printhead is moving in one direction (e.g., rightward). However, this solution significantly limits the throughput of the printer since it is necessary to wait for the printhead to return back to the starting position before printing another swath.
Another step that can be taken is to slow down the printing speed to give the ink/media interactions more time to stabilize between passes, but this too will significantly impact the printer throughput. Note that while this approach can help reduce the contributions of the bi-directional banding that are due to differences in the timing between ink laydowns, it will not alleviate the contributions that result from differences in ink ordering.
Another way to reduce the magnitude of the bidirectional banding artifacts is to increase the number of printing passes. This will effectively slow down the rate of ink deposition and dilute the impact of the differences in ink order and timing at any given location. However, this will also have a direct impact on the throughput of the printer, so it is not a desirable solution for applications where print speed is a critical requirement.
Another solution that has been proposed is to modify the amount of ink laydown for the rightward and leftward printing passes. One way to accomplish this is to use different color transforms to process the image for the different printing passes. For example, see U.S. Pat. Nos. 6,354,692 and 7,054,034, and U.S. Patent Application Publication No. 2003/0048327. Alternatively, an ink depletion operation can be used to modify the amount of ink that is printed depending on the print direction. One way that this can be accomplished is to modify the print masks as described in U.S. Pat. No. 6,545,773. These methods can help alleviate the component of the bi-directional banding that results from differences in ink ordering. However, they will be ineffective at compensating for the contributions of the bidirectional banding that result from differences in the timing between ink laydowns since these effects will vary from left to right across the page, and these methods do not provide for changing the ink laydown as a function of the horizontal position.